1. Field of the Invention
The present invention relates to a high-frequency signal generator for use in a millimeter wave radar device installed on a motor vehicle, and a radar module employing such a high-frequency signal generator.
2. Description of the Prior Art
Radar devices for use on motor vehicles such as automobiles in combination with warning units for preventing collisions are required to have a high degree of resolution for detecting objects in close distances of about several tens of centimeters. In view of such a high-resolution requirement, an FM radar is preferable to a pulse radar for use in the vehicle-mounted radar devices. Since the maximum range that may be detected up to a target such as a preceding motor vehicle or an upcoming motor vehicle is of a relatively short distance of about several hundred meters, it is suitable for such a radar to use radiowaves in the millimeter range which have a frequency of about 60 GHz and can be attenuated greatly upon propagation in order to prevent radiated radiowaves from being propagated beyond a necessary range and also from interfering with existing microwave communications equipment. Use of millimeter waves is also preferable from the viewpoint of reducing the size of a radar module including an antenna, FM signal generators in front and rear stages, a mixer, and other components.
Heretofore, FM radar modules in the millimeter range are constructed in the form of a microstrip line or a waveguide. Because the microstrip line radiates a large amount of power, it suffers a large loss and tends to cause interference between a plurality of modules, resulting in a reduction in measuring accuracy. The waveguide is disadvantageous in that its circuit is large in size and expensive.
One of the attempts to solve the above problems is a non-radiative dielectric (NRD) waveguide as disclosed in an article "Millimeter wave integrated circuit using a non-radiative dielectric waveguide" written by Yoneyama et al. and published in the Journal of Electronic Information Communications Society, Vol. J 73 C-1 No. 3 pp. 87-94, March 1990. The disclosed non-radiative dielectric waveguide comprises two confronting conductive plates spaced from each other by a distance slightly smaller than a half wavelength and a rod-shaped dielectric member inserted between the conductive plates for allowing only propagations along the rod-shaped dielectric member. The upper and lower surfaces of the non-radiative dielectric waveguide are completely shielded by the conductive plates. Since the distance between the conductive plates is shorter than the half wavelength, radiowaves are fully prevented from leaking laterally out of the non-radiative dielectric waveguide. Therefore, any power radiation from the non-radiative dielectric waveguide is very small, effectively avoiding radiation loss in a module and interference between modules.
Various components including a directional coupler and an isolator can easily be fabricated by positioning non-radiative dielectric waveguides closely to each other or adding ferrite. Therefore, modules employing non-radiative dielectric waveguides can be made smaller than the conventional microstrip arrangement where components are separately produced and interconnected by a waveguide. The above article also discloses small-size, high-performance transmitter and receiver structures for use in the millimeter wave band which employ non-radiative dielectric waveguides.
FIG. 10 of the accompanying drawings shows in cross section the structure of a conventional gunn oscillator for use as a high-frequency signal generator in a transmitter in the millimeter wave band. The conventional gunn oscillator comprises a gunn diode GD threaded in a diode mount DM. The distal end of a bias supply line B is fixedly bonded by silver paste to an upper conductor of the gunn diode GD which is exposed from a dielectric substrate D by cutting off a portion of the dielectric substrate D with a knife.
The high-frequency signal generator with the gunn oscillator shown in FIG. 10 cannot be fabricated with good reproducibility because the process of cutting off the dielectric substrate D with a knife and bonding the end of the bias supply line B to the conductor is relatively complex and time-consuming.
The frequency adjustment for the high-frequency signal generator disclosed in the above article is cumbersome as the oscillation frequency is adjusted by adjusting the dimensions of a metal foil oscillator.
While the above article shows the high-frequency signal generator using the gunn diode, it does not discuss any optimum arrangement for an FM signal generator for use in an FM radar module.
The article also discloses a gunn oscillator as shown in FIG. 11 of the accompanying drawings and a non-radiative dielectric waveguide for guiding signals which are generated in the millimeter wave band by the gunn oscillator to an antenna or the like. As shown in FIG. 11, the gunn oscillator and its surrounding circuits comprise upper and lower conductive plates 31, 32 serving as a non-radiative dielectric waveguide, a diode mount 33 sandwiched between the upper and lower conductive plates 31, 32, a gunn diode 34 threaded in the diode mount 33, a printed-circuit board 35 fixed to a side of the diode mount 33, a dielectric rod 40 for guiding a signal generated in the millimeter wave band by the gunn diode 34 to an antenna or the like (not shown), and a metal foil oscillator 41 for guiding the signal generated in the millimeter wave band by the gunn diode 34 to the dielectric rod 40.
In FIG. 11, the distance between the upper and lower conductive plates 31, 32 is set to a value slightly smaller than half the wavelength of the signals used in the millimeter wave band. If the signals have a frequency of about 60 GHz, for example, then the distance between the upper and lower conductive plates 31, 32, and hence the thickness of the diode mount 33 is of a small value of about 2.5 mm. Commercially available packaged gunn diodes are mounted on a heat-radiating stud which is of a diameter ranging from 3 to 4 mm. Therefore, it is necessary to machine them to make them ready for use in actual applications, as shown in FIGS. 12(A) and 12(B). First, as shown in FIG. 12(A), a gunn diode 35 is threaded in a metal block which is 5 to 6 mm thick. Then, as shown in FIG. 12(B), upper and lower portions of the metal block are cut off to reduce the thickness thereof to about 2.5 mm, and grooves dimensioned to a 1/4 wavelength are defined in the metal block to prevent the signals from leaking out. In this manner, the gunn diode 34 is mounted on the diode mount 33.
Inasmuch as the gunn oscillator requires complex machining as shown in FIGS. 12(A) and 12(B) to fabricate the diode mount 33 that is of a small thickness, the process of manufacturing the gunn oscillator is time-consuming, and the produced gunn oscillator is costly.
Furthermore, as shown in FIG. 13 of the accompanying drawings, the diode mount 33 has a recessed step on which an upper flange of the gunn diode 34 is placed. Since the depth .delta. of the recessed step is subject to variations, the height .epsilon. that a lower conductor of the gunn diode 34 projects from the surface of the diode mount 33 also suffers variations, resulting in varying oscillation characteristics.